This invention relates to processing of semiconductor wafers in a vacuum chamber and, more particularly, to methods and apparatus for thermal transfer in an ion implantation system which utilizes gas as a transfer medium.
In the fabrication of integrated circuits, a number of processes have been established which involve the application of high energy beams onto semiconductor wafers in vacuum. These processes include ion implantation, ion beam milling and reactive ion etching. In each instance, a beam of ions is generated in a source and directed with varying degrees of acceleration toward a target. Ion implantation has become a standard technique for introducing impurities into semiconductor wafers. Impurities are introduced into the bulk of semiconductor wafers by using the momentum of energetic ions as a means of imbedding them in the crystalline lattice of the semiconductor material.
As energetic ions impinge on a semiconductor wafer and travel into the bulk, heat is generated by the atomic collisions. This heat can become significant as the energy level or current level of the ion beam is increased and can result in uncontrolled diffusion of impurities beyond prescribed limits. As device geometries become smaller, this uncontrolled diffusion becomes less acceptable. A more severe problem with heating is the degradation of patterned photoresist layers which are applied to semiconductor wafers before processing and which have relatively low melting points.
In commercial semiconductor processing, a major objective is to achieve a high throughput in terms of wafers processed per unit time. One way to achieve high throughput in an ion beam system is to use a relatively high current beam. However, large amounts of heat may be generated in the wafer. Thus, it is necessary to cool the wafer in order to prevent elevated temperatures from being attained.
Techniques for keeping the wafer temperature below a prescribed limit have included batch processing, in which the incident power is spread over a number of wafers, time-shared scanning of the beam and conductive cooling through direct solid-to-solid contact between a wafer and a heat sink. The cooling efficiency of systems employing solid-to-solid contact is limited by the extent to which the backside of the wafer contacts the thermally conductive surface, since, at the microscopic level, only small areas of the two surfaces (typically less than 5%) actually come into contact.
The technique of gas conduction is known to permit thermal coupling between two opposed surfaces and has been applied to semiconductor processing in vacuum. In one approach, gas is introduced into a cavity between a wafer and a supportn plate. The achievable thermal transfer with this approach, however, is limited, since bowing of the wafer occurs at low gas pressures.
Gas-assisted, solid-to-solid thermal transfer with a semiconductor wafer is disclosed in U.S. Pat. No. 4,457,359, filed May 25, 1982, and assigned to the assignee of the present application. A semiconductor wafer is clamped at its periphery onto a shaped platen. Gas under pressure is introduced into the microscopic void region between the platen and the wafer. The gas pressure approaches that of the preloading clamping pressure without any appreciable increase in the wafer-to-platen spacing. Since the gas pressure is significantly increased without any increase in the wafer-to-platen gap, the thermal resistance is reduced; and solid-to-solid thermal transfer with gas assistance produces optimum results. In both of these approaches, the gas is supplied from a gas source, including means for regulating the pressure, coupled to the thermal transfer region behind the wafer.
As the demand for higher throughput ion implantation systems increases, it will become necessary to utilize higher currents, thereby requiring the application of gas cooling to batch systems. Typically, in batch systems, a number of wafers, for example 25, are mounted on a large disc which is rotated during ion implantation. The ion beam can be scanned across the rotating disc or the disc can be translated mechanically during rotation to provide uniform ion dosage over the surface of the wafers.
The use of gas cooling in a batch processing system is complicated by two factors. First, the hardware required to introduce gas into the thermal transfer region behind the wafer must be repeated at each wafer location. This greatly increases the complexity and cost of the system. In addition, connections external to the disc must be made through rotary connections along the axis of rotation. Prior art rotating discs have been water cooled with the cooling water piped to the disc through a rotating seal. The addition of connections for gas cooling would further complicate this arrangement.
It is an object of the present invention to provide novel apparatus for thermal transfer with a semiconductor wafer in vacuum.
It is another object of the present invention to provide novel methods and apparatus for gas conduction thermal transfer with a semiconductor wafer in an ion implantation system utilizing liquid cooling of a heat sink.
It is still another object of the present invention to provide novel methods and apparatus for gas conduction thermal transfer in an ion implantation system utilizing a movable element for mounting a plurality of wafers.
It is yet another object of the present invention to provide methods and apparatus for thermal transfer with a semiconductor wafer in a vacuum chamber which is vented during a portion of the processing cycle.